Jacqueline A. Hannam

Detecting atmospheric pollution in surface soils using magnetic measurements: a reappraisal using an England and Wales database.

Industrial activity such as burning of fossil fuels produces magnetically enhanced particulates. These particulates consist of coarse-grained multidomain and stable single domain magnetic minerals. Two threshold values of low field magnetic susceptibility (chi(LF)) and frequency dependent susceptibility percentage (chi(FD)%) discriminate ferrimagnetic minerals of these sizes and can act as a tracer of magnetic pollution. Application of the thresholds to a magnetic topsoil data set (n=5656 across England and Wales) revealed 637 samples potentially dominated by pollution particulates. The magnetic parameters of these samples display a negative correlation with distance to urban areas and positive correlations with metals associated with anthropogenic activity (Cu, Pb, and Zn). Results of experimentation with threshold values and modelling of magnetic anomalies suggest that regional factors such as geology and potential for pedogenic secondary magnetic enhancement should be considered when setting threshold values.

Testing competing hypotheses for soil magnetic susceptibility using a new chemical kinetic model

A chemical kinetic model is presented for the formation and accumulation of secondary ferrimagnetic minerals (SFMs) in soil constructed using experimentally determined rate constants and validated against field data. The primary objective is to critically assess the significance of competing causal mechanisms and disputed environmental controls under temperate conditions. Four findings are important in relation to current application of soil magnetic susceptibility data. First, transformation of hydrous ferric oxide to magnetite should dominate SFM formation, controlled primarily by parent material ferrous silicate concentration and climate. Second, abiotic reactions should account for most of the SFM production; the most significant impact of high Fe2+ concentrations created by dissimilatory iron-reducing bacteria is enhanced export of iron from the soil in runoff. Third, the model predicts a correlation between hematite and magnetite concentrations, weakening field support for direct transformation of hydrous ferric oxide to maghemite. Fourth, magnetic susceptibility enhancement should increase strongly with weathering duration.

Modeling soil magnetic susceptibility and frequency-dependent susceptibility to aid landmine clearance

Information on the electromagnetic properties of soils and their effects on metal detectors is increasingly necessary for effective demining due to limited detector efficacy in highly magnetic soils and the difficulty of detecting minimummetal mines. Magnetic measurements of soils, such as magnetic susceptibility and frequency dependent susceptibility can aid the detection of problem soils, but are not part of standard soil analyses. Consequently, little information about soil magnetism exists within the soil, environmental science and environmental geophysics communities. Lack of empirical data may be compensated through the estimation of soil magnetic characteristics by predictive modeling approaches. Initial modeling of soil types in Bosnia and Herzegovina (BiH) was attempted by expert and analogue approaches, using only coarse scale soil type information, which resulted in the production of national soil maps for low field and frequency-dependent susceptibility. Validation of the maps was achieved by comparison of empirical magnetic data from soil samples in the National Bosnian soil archive in Sarajevo. Discrepancies between the model and empirical data are explained in part by the differences in soil parent material within each soil type, which controls the amount of Fe released into the soil system available for in situ conversion to magnetic Fe oxides. The integration of soil information (type and parent material), expert knowledge and empirical data refines the predictive modeling of soil magnetic characteristics in temperate-Mediterranean environments such as BiH. Further spatial separation of soil types in the landscape can be achieved by digital terrain modeling. Preliminary fine-scale, landscape-soil modeling indicates improved spatial resolution of soil types compared with the original coarsely-mapped soil units, and the potential to synthesize local scale soil magnetic maps.

A synthesis of current knowledge and future directions for soil magnetism research

Magnetic properties of soils have adverse effects on metal detectors, particularly hampering operations during clearance of landmines and unexploded ordnance. Although there is well established research in soil magnetism and modeling electromagnetic induction systems these have tended to exist in disparate disciplines. Hence, a workshop was organized to bring together researchers, academics, stakeholders and manufacturers to discuss key priorities for research and technology in a unique multidisciplinary environment. Key knowledge gaps identified include limited information on the spatial heterogeneity of soil magnetic properties in 2D and 3D, whether current models describing soil responses are appropriate for all soils and the need for compensation mechanisms in detectors to be improved. Several priorities were identified that would maximize future developments for multidisciplinary research in soil magnetism and detector technology. These include acquiring well constrained empirical data on soil electromagnetic properties and detector response over the frequency range of detectors; development of predictive models of soil magnetic properties; investigating variability of soil magnetic properties in two and three dimensions across a range of scales. Improved communication between disciplines is key to effective targeting and realization of research priorities. Possible platforms include a multidisciplinary pilot study at an appropriate site and the development of an online repository to assist dissemination of results and information.